This invention relates to the loading of the mould forming cavity with powder or granular material, in ceramic tile manufacture.
The mould cavity is loaded with powder or granular material by a loading apparatus which releases the material into the cavity through its upper mouth.
Usually the loading apparatus comprises a loading carriage of horizontal flat form, having for each mould cavity a lowerly and upperly open loading compartment and provided with a suitable grid; each compartment corresponds to a mould cavity and has plan dimensions substantially equal to those of the cavity. The carriage is moved forwards and rearwards while slidingly resting on a flat continuous surface positioned as a continuation of the upper surface of the die plate containing the cavity to be filled.
The carriage is moved synchronously with the press operations, between a retracted position in which the loading compartment receives the loading material and an advanced position in which the compartment lies above the mould cavity so that the material falls into the cavity by gravity.
If several layers of different materials are to be arranged in the mould cavity, further means are also provided, for example several loading compartments for the same mould cavity, one for each material; or one or more hoppers are used, transported by the same carriage, each of which releases material through its lower mouth into the mould cavity.
In all cases, the material is deposited into the mould cavity while the means (compartments with grids and/or hoppers) from which the material falls move horizontally above the cavity. In addition, the lower edge of said means is necessarily maintained close to the plane of the upper mouth of the mould cavity and is moved flush with the upper mouth of the mould cavity, so that the upper surface of the material which falls into the cavity is flat or flush with the upper mouth of the cavity. The horizontal movement of said lower edges plus the action of an actual scraping means, with which the carriage is provided, produces a scraping action involving a small thickness of the upper surface layer of the material deposited into the mould cavity, but displacing the powders in a disordered manner, with the result that the original plan distribution of its particles, produced by the vertical descent of the material into the mould cavity, is completely changed.
If the loaded material is perfectly uniform in terms of its colour and particle size, this scraping action has no practical effect on the appearance of the material upper surface.
If however the material is not uniform, as in the case of bulk-coloured multi-colour tiles, which are composed of materials which differ in terms of colour and/or particle size and are present either as separate or partly mixed masses, the horizontal movement of the carriage produces in practice, on the upper surface of the material loaded into the cavity, an arrangement in which the powders form striations in the scraping direction, or a sort of patina covering the underlying powder distribution, with a resultant appearance much different from that which was required for the tile.
To overcome said drawback, the tile, after pressing, has to be subjected (sometimes before firing, but more generally after firing) to a smoothing action by which a thin surface layer is removed by abrasion, of such a thickness as to remove said effects produced by the scraping of the material loaded into the mould cavity and to expose the underlying original distribution of the material particles.
This smoothing action involves considerable cost, in terms of equipment, additional operations and longer production time.
Moreover, tiles cannot be produced in which the upper surface, obtained by pressing, has an uneven or embossed or relief-patterned appearance, because said smoothing action would damage such effects, and leave the tile upper surface flat.
An object of this invention is to provide a loading unit and relative method able to overcome said drawbacks, and in particular able to remove the described effects produced by said scraping action during the loading of the material into the mould cavity, without it being necessary to operate on the pressed or fired tile.
A further object of the invention is to enable a bulk-coloured multi-colour tile to be formed having exposed surfaces which are not flat.
A further object of the invention is to enable an extremely thin layer (or layers) of material to be loaded onto a base layer.
These and further objects are attained by the invention as characterised in the claims.